Liquid crystal display (LCD) modules are becoming more and more highly used in everyday life and work. To create images on LCD screens the LCD must be electrically connected to the display electronics to create a completed module. There are several typical techniques for electrically interconnecting the LCD and electronics, including; chip on glass (COG), where the display driver chip is physically bonded to the display's electrical traces on the display substrate. Another technique is to physically bond the display substrate to a flexible polyimide material, called a flex, which has embedded copper traces and pads. If the flex technique is used then the remaining unbounded end of the flex must be connected to a PCB or TAB that contains the display driver chip. An interconnect technique must be used for each of the display substrates, resulting in two bonding steps and two sets of driver chips for each display. It is possible to electrically connect one substrate's leads to the other substrate's leads (referred to as indium-tin oxide (ITO) traces, but can also be other transparent conductors such as conducting polymer, carbon nanotubes or others), which is referred to as an electrical cross-over. This is particularly useful when there are a smaller amount of total ITO traces for the LCD and then only one substrate requires physical bonding to either the COG or the flex. An electrical cross-over allows for a reduction of interconnect steps and materials used and thus, represents a cost reduction.
A common technique to electrically cross-over an LCD containing glue gasket around the perimeter is to place conductive spheres, or spacers, in that gasket. Therefore, the conductive spheres are held in one location in the display. In this situation ITO traces from both substrates touch the conductive spheres, allowing for an electrical connection between traces on both the substrates. This technique requires that the LCD uses a gasket around the perimeter.
Instead of electrically crossing-over one can also use one piece of flex to connect to both substrates. In this case the flex must have vias in it to allow connection to all traces on either the top or bottom side of the flex. Vias are in a simplistic sense a hole drilled in the polyimide that is then filled with copper, such that the copper filled hole has copper traces on both sides of the flex that come into contact with the via. This technique allows the display driver chips to be located on a PCB and requires fewer driver chips, thus reducing cost. However, a flex containing vias is typically very expensive.
To use as few display driver chips and interconnect processes for the least possible cost on a display that does not use a gasket around the perimeter a new type of electrical cross-over is required.